Traditionally, audio amplifiers have included a feedback mechanism that detects clipping or a certain distortion level at the amplifier output. The feedback mechanism transmits a detection signal to an earlier point in the signal path before an input to the amplifier, so that clipping can be avoided or a maximum distortion level can be maintained adaptively. The feedback clip detection signal is sometimes called “clip detect”, and is monitored continuously to control clipping and distortion. In digital signal processing (DSP) based amplifiers, the process of monitoring the clip detect is usually on a sample-by-sample basis, at the audio sample rate. This allows for a fast reaction time to react to clipping and to reduce the level of the input signal to the amplifier.
The processing architectures for DSP-based amplifiers are currently moving to using frame-based processing, where a function or set of functions are performed simultaneously on all audio samples of a set of contiguous audio samples. Such a set may be 128 samples long, for example. This processing approach is used because it allows processing efficiencies.
A problem with frame-based processing is that signals such as clip detect change much faster than the rate that frames are processed (the audio sample rate divided by the frame size). The state of the clip detect that is used as an input to processing within the frame can only be the state that exists at the frame boundaries. The processing within the frame includes processing to adjust the audio level to avoid clipping or excessive distortion, according to the state of clip detect. However, this process may not provide sufficient control of the audio level, since the clip detect is most likely changing faster than the value of clip detect used during the processing in the frame. Thus, excessive clipping and distortion may result or excessive reduction of the signal may result.
Currently, one method is to allow processing interrupts to occur when the clip detect indicates that clipping has occurred. By allowing processing interrupts, normal frame-based processing flow is broken, which causes significant processing inefficiency.
Various algorithms exist for controlling clipping and distortion. These algorithms process the signal prior to its input to the amplifier. A class of algorithms may be described as “predictive”, where the algorithm attempts to predict when clipping or excessive distortion will occur, such prediction being performed without the aid of a clip detect feedback signal. These algorithms may be described as “feed-forward”, since they do not use feedback. These algorithms depend on using an accurate Vout vs. Vin or distortion vs. Vin transfer function model of the amplifier so that its point of clipping or points of certain distortion levels are known beforehand. This transfer function must be characterized during the product design process, and therefore must be stable with respect to temperature, voltage, and design changes due to other factors in the development process. This stability may not be possible, expensive or inconvenient.
In one implementation, for control of the maximum audio input level to an amplifier to control clipping and distortion when no clip detect signal is available, the control method is based on a transfer function determined when the particular copy of the amplifier is built and tested. Such calibration steps add cost to produce the unit as well as to prove-out and maintain the calibration process.
In another implementation, the transfer function is determined when the amplifier is designed, based on analysis of all possible variations of the transfer function due to parts and production variations. Design margin may be added. This approach either adds cost to the design, by using tighter-tolerance components and manufacturing so that a maximum desired power output is available, or this approach forces the maximum desired power output to be reduced. Amplifier power output is a key customer characteristic and is typically rated by third-parties in the industry for use by consumers.
It would be desirable to have a feedback calibration system and a method for controlling an electronic signal, wherein the feedback calibration system and the method militate against an undesirable condition of the electronic signal during a signal processing by incorporating a real-time transfer function estimate in the processing of the electronic signal.